Paddlewheel-type marine speed sensors are known in the art. There are typically two types of speed sensors, a transom mount speed sensor and a thru-hull speed sensor. The transom mounted speed sensor mounts to the transom of a marine vessel, while the thru-hull speed sensor mounts through an opening in the hull of the marine vessel. Each type of these sensors have paddles or blades which are asymmetric in shape and formed of magnetized amorphous magnetic material, such as barium ferrite. In some embodiments, these speed sensors can be combined with a depth sensor, a temperature sensor, or a combination of both depth and temperature.
In operation, as the marine vessel-traverses a body of water the paddlewheel rotates about an axis which is transverse the direction of travel. A change in the magnetic field emanating from the magnetized paddles is sensed by a coil or Hall-effect device located adjacent the paddlewheel. Upon sensing the change in the magnetic field, the Hall-effect device generates an electrical signal whose frequency is directly proportional to the rotational speed of the paddlewheel which in turn should be the same as the speed of the vessel. The vessel's speed is then displayed to the vessel's operator, typically in the cockpit of the vessel's helm.
One problem with paddlewheel-type marine sensors is they are inherently non-linear devices. There are several causes of non-linearity, for example; 1) as the vessel speed decreases and the distance from the “leading edge” increases, the boundary layer near the marine sensor becomes thicker causing the sensor to measure less than the true speed of the vessel. 2) the drag induced by fluid recirculation in the paddlewheel cavity is also not linear with speed since the force applied to the paddles is proportional to the square of the speed; 3) rotational friction from the shaft bearings supporting the paddlewheel is also not linear; and 4) the angle of attack of the vessel hull changes with speed, that is the angle at which the hull cuts through a body of water causing the sensor to misread the actual speed; 5) hull displacement changes with speed, that is the portion of the hull that is in the water changes with speed, i.e., as the vessel's speed increases less hull remains in the water and when the vessel's speed decreases more hull is in the water.
Various techniques have been devised in an attempt to improve the high-speed performance of paddlewheel speed sensors. One such technique uses an open paddlewheel construction, in which four paddles are each supported by arms which define an open space between the paddle and the wheel shaft. Another such technique employs a waterwheel having a plurality of buckets, closed at their side, and recessed in the peripheral edge of the wheel. The slot within which the wheel rotates is open on the downstream side. This provides a relief space which prevents a buildup of pressure on the downstream side, which would vary the rotational characteristics of the wheel in a non-linear manner.
Another problem with paddlewheel speed sensors is that a significant amount of “jitter” exists in the paddlewheel rotational speed. Jitter is an oscillation in rotational speed attributable to the turbulent flow present in the paddlewheel cavities at nearly all speeds, and is even present when the vessel velocity is quasi-constant.